Light bulb



FIG. 1 is a perspective view of my new ornamental design for a lightbulb;

FIG. 2 a right side elevational view thereof, the left elevational viewbeing the same;

FIG. 3 is a front side elevational view thereof, the rear elevationalview being the same;

FIG. 4 is a top plan view thereof; and,

FIG. 5 is a bottom plan view thereof.

The broken lines showing environment in the drawings are forillustrative purposes only and form no part of the claimed design.

The ornamental design for a light bulb, as shown and described.